The proliferation and differentiation of B cells is a complex process directed and regulated through interactions with many other cell types. Among the B cell-specific molecules involved in this process, CD22 is believed to serve a significant role since it is an adhesion molecule of B cells that may function in homotypic or heterotypic interactions (Stamenkovic et al., Nature 344:74 (1990); Wilson et al., J. Exp. Med. 173:137 (1991); Stamenkovic et al., Cell 66:1133 (1991)). The CD22 protein is expressed in the cytoplasm of progenitor B and pre-B cells (Dorken et al., J. Immunol. 136:4470 (1986); Dorken et al., "Expression of cytoplasmic CD22 in B-cell ontogeny. In Leukocyte Typing III. White Cell Differentiation Antigens. McMichael et al., eds., Oxford University Press, Oxford, p. 474 (1987); Schwarting et al., Blood 65:974 (1985); Mason et al., Blood 69:836 (1987)), but is found only on the surface of mature B cells, being present at the same time as surface IgD (Dorken et al., J. Immunol. 136:4470 (1986)). CD22 expression increases following activation and disappears with further differentiation (Wilson et al., J. Exp. Med. 173:137 (1991); Dorken et al., J. Immunol. 136:4470 (1986)). In lymphoid tissues, CD22 is expressed by follicular mantle and marginal zone B cells, but only weakly by germinal center B cells (Dorken et al., J. Immunol. 136:4470 (1986); Ling et al., "B-cell and plasma antigens: new and previously defined clusters" In Leukocyte Typing III. White Cell Differentiation Antigens, McMichael et al., eds., Oxford University Press, Oxford, p. 302 (1987)). However, in situ hybridization reveals the strongest expression of CD22 mRNA within the germinal center and weaker expression within the mantle zone (Wilson et al., J. Exp. Med. 173:137 (1991)). CD22 is probably involved in the regulation of B cell activation since the binding of CD22 mAb to B cells in vitro has been found to augment both the increase in intracellular free calcium and the proliferation induced after crosslinking of surface Ig (Pezzutto et al., J. Immunol. 138:98 (1987); Pezzutto et al., J. Immunol. 140:1791 (1988)). Other studies have determined, however, that the augmentation of anti-Ig induced proliferation is modest (Dorken et al., J. Immunol. 136:4470 (1986)). CD22 is constitutively phosphorylated, but the level of phosphorylation is augmented after treatment of cells with PMA (Boue et al., J. Immunol. 140:192 (1988)). Furthermore, a soluble form of CD22 inhibits the CD3-mediated activation of human T cells, suggesting CD22 may be important in T cell-B cell interactions (Stamenkovic et al., Cell 66:1133 (1991)).
cDNA that encode the CD22 protein have been isolated by two different research groups, revealing the protein to be a member of the Ig-superfamily homologous with myelin-associated glycoprotein (MAG), carcinoembryonic antigen (CEA), and neural-cell adhesion molecule (N-CAM) (Stamenkovic et al., Nature 344:74 (1990); Wilson et al., J. Exp. Med. 173:137 (1991)). The first CD22 cDNA isolated encodes a protein with 5 extracellular Ig-like domains that mediates monocyte and erythrocyte attachment to COS cells transfected with the cDNA (Stamenkovic et al., Nature 344:74 (1990)). A second isolated CD22 cDNA encodes an extracellular region of 7 Ig-like domains and a cytoplasmic tail having a different COOH sequence that is 23 amino acids longer than the COOH sequence of the first cDNA isolate (Wilson et al., J. Exp. Med. 173:137 (1991)). This full-length CD22 cDNA encodes a protein with a single NH.sub.2 -terminal V-like domain and 6 C-type domains, which mediates the binding of T and B lymphocytes to transfected COS cells (Stamenkovic et al., Nature 344:74 (1990); Wilson et al., J. Exp. Med. 173:137 (1991)). In vitro translation of a full-length CD22 cDNA generates a 95,000 M.sub.r protein, and the predicted extracellular portion of the molecule has 12 N-linked glycosylation sites (Wilson et al., J. Exp. Med. 173:137 (1991)), which is consistent for a protein of .about.140,000 M.sub.r. It has been reported that the 7 Ig-like domain species of CD22 is a B cell-specific ligand for CD45RO on T lymphocytes and a receptor for .alpha.2,6-sialyltransferase, CDw75, on B lymphocytes (Stamenkovic et al., Cell 66:1133 (1991)).
Competitive binding inhibition studies using .sup.125 I-labelled prototype mAb in a cellular radioimmunoassay (CRIA) on cell line JOK1 have revealed five different epitopes recognized by 12 tested anti-CD22 monoclonal antibodies. Two independent epitopes are represented by mAb HD39, HD239, S-HCL1, and BL9 (epitope A) and OTH228 (epitope E). Three other epitopes represented by mAb HD6 (epitope B), mAb To15, G28-7 (epitope C), and mAb BL-3C4 (epitope D) seemed to be closely related to each other because some mAb showed overlapping reactions. Antibodies OM-124 and 3G5 reacted both with epitopes B and C whereas mAb IS7 reacted likewise with epitopes B and D (Schwartz-Albiez et al., "The carbohydrate moiety of the CD22 antigen can be modulated by inhibitors of the glycosylation pathway." In Leukocyte Typing IV. White Cell Differentiation Antigens. Knapp et al., eds., Oxford University Press, Oxford, p. 65 (1989)).
COS cells transfected with a CD22 cDNA lacking Ig-like domains 3 and 4 have been reported as expressing CD22 epitopes A and D and as lacking epitopes B, C and E (Stamenkovic et al., Nature 344:74 (1990)). In rosetting assays using this cDNA to transfect COS cells, mAb that bind to epitope A (S-HCL1) blocked RBC binding while mAb binding to epitope D (BL-3C4) did not block. In contrast, preincubation of transfected COS cells with either anti-epitope A (S-HCL1) or anti-epitope D (BL-3C4) mAb failed to block monocyte cell adhesion, but when both antibodies were used in conjunction, partial blocking was observed. These results suggested that different epitopes of CD22 participate in erythrocyte and monocyte adhesion and that different ligands may be recognized by each epitope (Stamenkovic et al., Nature 344:74 (1990)).
Additional mAb exhibiting an ability to completely block CD22 binding to all leukocyte types would clearly be advantageous. Such mAb could be used in therapeutic methods for treating patients to retard or block B cell function, particularly in autoimmune disease.